This invention relates to pulsed droplet deposition apparatus and more particularly to such apparatus including a plurality of droplet deposition channels. Typical of this kind of apparatus are multi-channel pulsed droplet ink jet printers often also referred to as "drop-on-demand" ink jet printers. In contrast to complexities inherent in other printing systems such as electrostatic, magnetic, electrophotographic, thermal or ion projection, drop-on-demand ink jet printers offer a simple approach to electronically controlled printing with the advantage that the technique is non-contrasting and capable of high speed. It also places the minimum constraints on the ink formulation and printing surfaces.
Attempts have been made to produce multi-channel ink jet printers using piezo-electric actuators and reference is made in this connection to U.S. Pat. Nos. 3,946,398, 3,683,212, 3,747,120, 4,525,728; 4,549,191 and 4,584,590 and IBM Technical Disclosure Bulletin Vol. 23 No. 10 March 1981. According to this art, an ink channel connects an ink reservoir to an ejection nozzle. Piezo-electric transducers adjacent to the channel respond to a voltage impulse to generate a pressure pulse in the ink for ejecting an ink droplet from the nozzle. Piezo-electric actuators have the advantage of low energy requirement and this general approach has proved satisfactory for single nozzle printheads. It has not, however, proved practical for multi-channel printheads where a row of nozzles are to be operated at a relatively high nozzle density. One reason for this is that the piezo-electric transducers supply only limited movement and a relatively large active area, as compared with the nozzle aperture, is required to accomplish sufficient fluid displacement. In addition, the designs of piezo-electrically actuated printheads proposed in the art have not proved amenable to micro-fabrication and are expensive when manually assembled.
A further existing technology for the production of multi-channel drop-on-demand ink jet printers is known from, for example, U.S. Pat. No. 3,179,042; GB-A-2 007 162 and GB-A-2 106 039. These patent specifications disclose thermally operated printheads which, in response to an electrical input signal, generate a heat pulse in selected ink channels to develop a vapour bubble in the ink of those selected channels. This in turn generates a pressure pulse having the pressure and time characteristics appropriate for the ejection of an ink droplet from a nozzle at the end of the channel.
Thermally operated printheads of this nature possess a number of significant disadvantages. First, the thermal mode of operation is inefficient and typically requires 10 to 100 times the energy to produce an ink droplet as compared with known piezo-electric printheads. Second, difficulties are found in providing the very high levels of reliability and extended lifetimes which are necessary in an ink jet printhead. For example, thermal operated printheads have a tendency for ink deposits to form on the heating electrodes. Such deposits have an insulating effect sufficient to increase substantially the electrical pulse magnitude necessary to eject an ink droplet. Thermal stress cracks and element burnout, as well as cavitation erosion, have also proved difficult to eliminate. Third, only ink specifically developed to tolerate thermal cycling can be used and suitable ink formulations often prove to be of low optical density.
A still further technology for a multi-channel ink jet printer is disclosed in U.S. Pat. No. 4,023,180. This relies upon an electrodynamically generated pressure pulse in electrically conductive ink. A magnetic field is applied to the channels and electrodes positioned to enable a current to be passed through the ink in the selected channel. This proposal is not regarded as commercially practical. The resistance of the electrically conductive ink in the channel is found in practice to be comparable with the element resistance in a thermally operated printer as described above. As a consequence, when a current pulse is delivered sufficient to generate an electromagnetic pressure pulse in the ink and therefore expel a droplet, both vapour and electrolytically generated bubbles are also formed and sustained operation has proved impossible.
Reference is further directed to IBM Technical Disclosure Bulletin Vol. 18, No. 7, December 1975. this discloses a multi-nozzle ink jet printhead comprising a stack of wafers positioned between the pole pieces of a magnet. Each wafer provides a nozzle communicating with a funnel-shaped pump chamber. Electrodes project into the pump chamber and are in contact with mercury pellet. Application of a current to the electrodes of any selected wafer causes the associated mercury pellet to be driven towards the nozzle by electrodynamic action causing an ink droplet to be ejected from the nozzle.
If it is desired using this technique to produce an ink jet with a relatively large number of nozzles, bearing in mind that the corresponding wafers are to be accommodated between the pole pieces of a magnet, the gap between the pole pieces would become relatively large. It will be recognised that if a uniform field is to be assured over a wide pole gap, relatively large pole pieces are required; otherwise, edge effects will dominate.